I am trying to reload a captcha

Hello,

I am trying to reload a captcha with a button and javascript. The way I have it set up right is like this:

the HTML img
<img id="imgCaptcha" src="/php/create_captcha.php" />

the HTML button
<img id="captcha_img" src="/images/buttons/reload.png" class="neutral-link" rel="Reload" title="Reload" onClick="reloadCaptcha();">

the javascript
function reloadCaptcha() {
      if (refresh_count == '') {
            refresh_count = 0;
      }
      document.images["imgCaptcha"].src = '/php/create_captcha.php?c=' + refresh_count;
      refresh_count = refresh_count + 1;
}

the PHP
 //Start the session so we can store what the security code actually is
session_start();

//Send a generated image to the browser
create_image();
exit();

function create_image()
{
    $increment = $_GET['c'];
   
    //Let's generate a totally random string using md5
    $md5_hash = md5(rand(0,999)*$increment);
    //We don't need a 32 character long string so we trim it down to 5
    $security_code = substr($md5_hash, 15, 5);

    //Set the session to store the security code
    $_SESSION["security_code"] = $security_code;

    //Set the image width and height
    $width = 200;
    $height = 27;

    //Create the image resource
    $image = ImageCreate($width, $height);  

    //We are making three colors, white, black and gray
    $white = ImageColorAllocate($image, 255, 255, 255);
    $black = ImageColorAllocate($image, 51, 79, 62);
    $grey = ImageColorAllocate($image, 0, 204, 102);

    //Make the background black
    ImageFill($image, 0, 0, $black);

    //Add randomly generated string in white to the image
    ImageString($image, 5, 100, 6, $security_code, $white);

    //Throw in some lines to make it a little bit harder for any bots to break
    ImageRectangle($image,0,0,$width-1,$height-1,$grey);
    imageline($image, 0, $height/2, $width, $height/2, $grey);
    imageline($image, $width/2, 0, $width/2, $height, $grey);
 
    //Tell the browser what kind of file is come in
    header("Content-Type: image/jpeg");

    //Output the newly created image in jpeg format
    ImageJpeg($image);
   
    //Free up resources
    ImageDestroy($image);
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